Overview Winter Climate for NH

cohen-schematic2

The diagram represents how Dr. judah Cohen pictures the Northern Hemisphere wintertime climate system.  He leads research regarding Arctic and NH weather patterns for AER.  He explains the dynamics in an interview at Washington Post (here):

My colleagues, at AER and at selected universities, and I have found a robust relationship between two October Eurasian snow indices and the large-scale winter hemispheric circulation pattern known as the North Atlantic or Arctic Oscillation pattern (N/AO).

The N/AO is more highly correlated with or explains the highest variance of winter temperatures in eastern North America, Europe and East Asia than any other single or combination of atmospheric or coupled ocean-atmosphere patterns that we know of. Therefore, if we can predict the winter N/AO (whether it will be negative or positive) that provides the best chance for a successful winter temperature forecast in North America but certainly does not guarantee it.

[Of the two indices we’ve analyzed], the first and longer [more data points] index is simply the monthly mean snow cover extent (SCE) for the entire month [of October] as measured from satellites. This record dates back to at least 1972 and is available on the Rutger’s Global Snow Lab website.

The second index that we developed last year, with the support of NSF and NOAA grants, measures the daily rate of change of Eurasian snow cover extent also during the entire month of October, which we refer to as the Snow Advance Index or SAI.

There have been recent modeling studies that demonstrate that El Nino modulates the strength and position of the Aleutian Low that then favors stratospheric warmings and subsequently a negative winter N/AO that are consistent with our own research on the relationship between snow cover and stratospheric warmings. So the influence of ENSO on winter temperatures in the Mid-Atlantic and the Northeast may be greater than I acknowledge or that is represented in our seasonal forecast model.

How It Works

Conceptual model for how fall snow cover modifies winter circulation in both the stratosphere and the troposphere–The case for low snow cover on left; the case for extensive snow cover on right.

1. Snow cover increases rapidly in the fall across Siberia, when snow cover is above normal diabatic cooling helps to;
2. Strengthen the Siberian high and leads to below normal temperatures.
3. Snow forced diabatic cooling in proximity to high topography of Asia increases upward flux of energy in the troposphere, which is absorbed in the stratosphere.
4. Strong convergence of WAF (Wave Activity Flux) indicates higher geopotential heights.
5. A weakened polar vortex and warmer down from the stratosphere into the troposphere all the way to the surface.
6. Dynamic pathway culminates with strong negative phase of the Arctic Oscillation at the surface.

From Eurasian Snow Cover Variability and Links with Stratosphere-Troposphere
Coupling and Their Potential Use in Seasonal to Decadal Climate Predictions by Judah Cohen

Extensive 2016 Siberian snowfall led to unusually rapid recovery of Arctic sea ice following relatively low September 2016 minimum.

What About Winter 2017-2018?

Dr. Cohen’s Winter Outlook for NH  September 18, 2017

Many important markers are currently being set indicating the atmosphere is beginning in earnest the transition from summer to winter. There are four features that I am monitoring closely over the coming weeks and months to gauge the evolving atmospheric circulation pattern and resultant weather across the NH.

The first is the nascent stratospheric polar vortex (PV). The PV has returned to the NH polar stratosphere. Much recent research including my own has shown that the relative strength of the PV if not forces, certainly leads prolonged periods of temperature anomalies across key regions of the NH. A strong PV is related to relatively milder temperatures across the mid-latitudes of the NH while a weak PV is related to relatively colder temperatures across the mid-latitudes of the NH. This relationship is strongest in mid-winter. Early signs are that the PV will start off relatively weak similar to last fall. This is somewhat surprising because increasing greenhouse gases favor colder stratospheric temperatures and hence a stronger PV. Poleward heat flux or vertical wave activity flux is predicted to be unusually active in the coming two weeks, which is likely the reason for the predicted relatively weak start to the PV.  (my bolds)

The active poleward heat flux is also likely related to the second feature that I will be following – high latitude blocking. The negative AO state is often a manifestation of strong high latitude blocking while the positive AO often reflects a lack of high latitude blocking. The predicted negative AO in the coming two weeks is a result of predicted strong high latitude blocking with the dominant block predicted to reside in the region of Scandinavia and the Barents-Kara seas. In the near term this will lead to a cold and snowy period across most of Siberia. Blocking in this region is favorable for weakening the stratospheric polar vortex and will likely lead to weakening of the PV over the next two weeks. If similar blocking occurs later on during the late fall and early winter it will favor a sudden stratospheric warming (SSW). SSW in the winter often precedes extended periods of severe winter weather across the continents of the NH. (my bolds)

The third feature is Arctic sea ice extent. The minimum in Arctic sea ice extent is achieved this time of year and if the minimum has not already been reached it should occur relatively soon. The past two blogs I suggested the possibility that the sea ice minimum could be similar to the years 2008 and 2010 and that is looking likely. Sea ice extent is extremely low compared to climatology but will not be a new record low. The largest anomalies are in the North Pacific side of the Arctic in the Beaufort Sea. This pattern matches recent Septembers. Typically, the largest anomalies migrate with the progression of fall to the North Atlantic side of the Arctic. It is my opinion that low sea ice favors high latitude blocking but the nature of the blocking is regionally dependent. For example, low sea ice in the Barents-Kara Seas favors blocking in the northwest Eurasia sector resulting in cold temperatures in parts of Asia. (my bolds)

The fourth feature is Siberian snow cover. My, along with my colleagues and others, research has shown that extensive Siberian snow cover in the fall favors a trough across East Asia with a ridge to the west near the Urals. The atmospheric circulation pattern favors more active poleward heat flux, a weaker PV and cold temperatures across the NH. With a predicted strong negative AO in the coming weeks, snow cover is likely to advance relatively quickly heading into October. It is very early in the snow season but recent falls have been snowy across Siberia and therefore I do expect another upcoming snowy fall across Siberia. Though admittedly, recent Siberian snow cover as a predictor of winter temperatures has been mixed.

Summary

Uh oh.  Now where did I put away my long johns?

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One comment

  1. Sara Hall · September 20, 2017

    I think it’s going to be cold this Northern winter, Ron. I can feel it in my bones…that and all the Finns, Swedes, Danes, Estonians and Russians we met this past Baltic “summer” who have told us so!

    Like

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